1. Field of the Invention
The present invention relates to rotating machinery balancing technology and, more particularly, to an eddy-current actuated balancer for rotating machinery.
2. Description of the Related Art
Under the industry trend of searching for high efficiency, the speed of rotating machinery has been greatly increased. In consequence, the problems of deterioration of processing precision, shortening of component service life and machine safety resulted from mass unbalance of spindle have become important issues to be discussed.
For example, the operating speed of the workpiece rotating shaft and cutting tool rotating spindle of machine tools has been greatly improved and the requirements for processing precision have become more critical. In consequence, spindle balancing techniques have become much more emphasized in the industry. Spindle balancing techniques include two types, i.e., the passive type and the active type. The most commonly seen passive type balancers are ball balancers. Fluidic balancers and electromagnetic-actuated balancers are the most commonly seen active type balancers.
A passive type ball balancer has the advantage of simple structure and does not require any actuator. However, a passive type ball balancer can simply be used in the conditions where the rotating speed is constant and above the resonance frequency of the spindle, for example, for use in CD-ROM dynamic balancing. Active type balancers require an extra actuator to move the counterweight to a predetermined angle so as to achieve balancing. Active type balancers are practical for use in a condition where the rotating speed is to be changed subject to different working conditions, for example, the balancing of the spindle of a machine tool or the transmission shaft of a car. An active type fluid balancer causes flowing of a fluid to a predetermined area subject to heating, thereby adjusting the position of the counterweight. A fluid balancer has the disadvantages of low reaction speed and low precision level. These drawbacks limit the application of a fluid balancer in a high speed spindle for high precision processing. Further, U.S. Pat. No. 5,757,662 discloses an electromagnetically actuate machine balancer, entitled “Electromagnetically actuated rotating machine unbalance compensator”. This design pertains to a matured active type dynamic balancing technique. However, because of the use of high precision bearings and permanent magnet arrays, this technique has the drawbacks of expensive cost and large dimension. During installation, the balancer cannot be kept inside the spindle and must be hung on the outside of the spindle.
FIGS. 6 and 7 show an eddy-current actuated balancer according to the prior art. According to this design, a rotary disc (or ring) 93 is mounted on a spindle 91 and partially inserted into an air gap in an electro-magnet 99. The rotary disc 93 is secured to the spindle 91 by means of a friction force. When not actuated by an eddy current, the rotary disc 93 is synchronously rotated with the spindle 91. However, when a magnetic field component is perpendicularly acted upon the surface of the rotary disc 93, the magnetic field will induce an eddy current on the rotary disc 93, and the cross product of the induced eddy current and the magnetic field will cause formation of an actuation force on the rotary disc 93 in a tangent direction reversed to the direction of rotation. When this actuation force overcomes the friction force between the spindle 91 and the rotary disc 93, the rotary disc 93 will be forced to make an angular displacement relative to the spindle 91, i.e. the spindle 91 and the rotary disc 93 will not rotate synchronously. By means of the aforesaid eddy current actuation principle, applying a pulse current to the electro-magnet 99 to generate a pulsed magnetic force on the rotating rotary disc 93 can induce a pulsed eddy-current actuation force on the surface of the rotary disc 93. When this pulsed eddy-current actuation force overcomes the friction force between the spindle 91 and the rotary disc 93, the rotary disc 93 will be forced to make an angular displacement relative to the spindle 91. In FIG. 7, the imaginary circles indicate an eddy current; the arrowhead direction indicates the current flowing direction. This pulsed eddy-current actuated balancing technique has the advantage of capable of driving the rotary disc 93 to make an angular displacement relative to the spindle 91 without touching the rotary disc 93. When compared to U.S. Pat. No. 5,757,662, this pulsed eddy-current actuated balancing technique has simple structure and small size characteristics.